a. Field of the Invention
This invention pertains to the fabrication of piezoelectric resonators which resonate at microwave frequencies. More particularly this invention pertains to the fabrication of multiple resonators on a single piezoelectric substrate, where the resonant frequency of one resonator is shifted by a small amount relative to the resonant frequency of one or more of the other resonators on the same substrate.
b. Description of the Prior Art
In the prior art, piezoelectric resonators, or "crystals" for use at high frequencies, e.g. 2 to 30 mhz, have been fabricated from thin slabs of quartz crystal. A conducting electrode is placed on the top surface of the slab and a second conducting electrode is placed on the bottom surface of the slab. Because the application of an electric field between the two electrodes causes the thin slab of piezoelectric material to deform mechanically, and because the periodic deformation of the thin slab of piezoelectric material exhibits a mechanical resonance, the attached pair of electrodes exhibit a similar electrical resonance.
The resonant frequency of the device may be raised by using an abrasive in a grinding or lapping process to reduce the thickness of the piezoelectric slab and thus raise the frequency of its mechanical resonance. However, at microwave frequencies, the piezoelectric slab is too thin to withstand grinding or lapping and often breaks. Furthermore, if the thin slab is supported by additional structure at its periphery, e.g. an inverted mesa structure, even if the thin slab does not break, the grinding or lapping pressure bends the slab and produces a slab having a non-uniform thickness, which non-uniformity substantially degrades the operation of the resonator.
In order to obtain a thin piezoelectric substrate having a uniform thickness, a thin substrate has been fabricated using etching processes or deposition processes using sputtering or evaporation techniques. Metal electrodes are then placed on the surfaces of the substrate using similar processes. See "High-Q Microwave Acoustic Resonators and Filters," by Lakin, Kline and McCarron, IEEE Trans. on Microwave Theory and Techniques, Vol. 41, No. 12, December 1993, p. 2139. Various methods have been used to fabricate such devices for use at microwave frequencies, see e.g. Guttwein, Ballato and Lukaszek, U.S. Pat. No. 3,694,677. The substrate may consist entirely of a piezoelectric material, or may consist of layers of piezoelectric and non-piezoelectric materials. See e.g. "Acoustic Bulk Wave Composite Resonators", Applied Physics Letters 38(3) by Lakin and Wang, Feb. 1, 1981.
Many techniques exist for fabricating piezoelectric resonators. For some applications a suitable resonator can be fabricated simply by adding conducting electrodes to a thin piezoelectric crystal "blank" obtained from commercial sources which "blank" may have been further thinned by processing. Another technique is to first fabricate a bottom electrode on a supporting substrate such as silicon. Next, a thin film of piezoelectric material is deposited over the electrode and substrate. The supporting substrate is then removed in some regions so as to expose the bottom electrode, which leaves the electrode and piezoelectric film in the form of a membrane or plate supported at the edges. The top electrode is then fabricated on the top surface of the membrane. The equivalent of a thin membrane may also be created by fabricating a sequence of quarter-wavelength thick layers of material upon a suitable substrate. A bottom electrode is then fabricated upon the uppermost quarter-wavelength reflector followed by a layer of piezoelectric material and finally by the top electrode. The quarter-wavelength thick layers of material act as reflectors and mechanically isolate the acoustic motion of the bottom electrode and of the piezoelectric material from the underlying substrate.
Curran et al., U.S. Pat. No. 3,222,622, have disclosed the fabrication of a plurality of resonators located upon a single substrate and electrically interconnected so as to provide complex filtering properties. Curran, et al. also disclose using different thicknesses for the metal electrodes on the different, resonators so as to obtain slightly different resonant frequencies for such resonators located upon a single substrate. Black et al., in U.S. Pat. No. 4,320,365 discloses various means for fabricating thin substrates that include a piezoelectric layer for use in making resonators. Black et al. disclose that the placement of acoustic absorbing material at the periphery of the electrodes and the removal of zinc oxide at the periphery of the electrodes may serve to enhance the resonance "Q" factor, reduce unwanted sidelobe response, and/or improve filter efficiency.
Roberts et al., U.S. Pat. No. 4,833,430, discloses the use of small coupling adjust spots to alter the resonant properties of coupled resonators located upon a single substrate. Roberts et al. also encountered some problems arising from errors in the alignment of successive masks used in the metal deposition process and they adjusted the thickness of the deposited metal to compensate for some of the consequences of the alignment errors.
As indicated above, it is known in the prior art (e.g. U.S. Pat. No. 4,320,365) that two resonators may be fabricated, upon a single substrate and that the two resonators can be made to have different resonant frequencies by fabricating the metal electrodes so that one of the electrodes forming one resonator has a thickness that differs from the corresponding electrode for the other resonator. It is also known in the prior art that one can. fabricate such electrodes having differing thicknesses by depositing each electrode in a separate step in the fabrication process. For example, referring to FIGS. 1 and 2, substrate 1, which may consist solely of a piezoelectric material or of layers of piezoelectric and non-piezoelectric materials, may be fabricated by any of the methods known in the prior art. Then by suitable masking operations, conducting electrodes 2 and 9 are placed by deposition, sputtering, or other means upon bottom surface 3 of the substrate. By means of suitable masking operations, conducting electrode 4 is deposited upon top surface 5 of the substrate. The area of electrode 4 that overlaps with the area of electrode 2 defines the physical location and extent of resonator 6. By suitable masking operations, electrode 7 is then placed upon top surface 5 of the substrate and the area of electrode 7 that overlaps with electrode 9 similarly defines resonator 8. Electrode 7 can be fabricated to have a thickness that is greater than the thickness of electrode 4 simply by increasing the length of time of the deposition or sputtering process that is used to fabricate electrode 7 as compared with the length of time used for the fabrication of electrode 4.
However, the practical problem with fabricating electrodes 4 and 7 in two completely separate steps is that it is very difficult to control accurately each of the two separate deposition or sputtering processes so as to obtain a thickness for electrode 7 that is greater than that of electrode 4 by only a small, controlled amount so as to obtain two resonators whose resonant frequencies differ from each other by only a small and controlled amount. For example for a resonant frequency of 1900 mhz., the resonators may comprise a piezoelectric film having a thickness of 1 micron and electrodes having a thickness of only 1000 angstroms (0.1 microns). An increase in the metal thickness of one electrode by 76 microns would reduce the resonant frequency by approximately 38 mhz. Accordingly, the amount of additional metal deposited would have to be controlled to an accuracy of 7.6 microns if one wished to obtain the specified frequency shift with an accuracy of 10 percent.
A similar problem arises in the fabrication of a resonator having its resonance at a particular specified frequency with high accuracy. It is difficult to control the thicknesses of the substrate and of the metal electrodes with enough accuracy to obtain the desired result.